1. Technical Field
This invention relates to photoinitiator systems for use in addition, i.e., free radically-initiated, polymerization.
2. Discussion of Prior Art
Addition polymerization (also referred to as free radical polymerization) of vinyl monomers by exposure thereof to radiation from the ultraviolet or visible portion of the electromagnetic spectrum is well known in the art. Polymerization of this type is also known as photopolymerization, photocuring, or curing.
Aryliodonium salts have been previously described for use as photoinitiators in addition-polymerizable compositions. References relating to such compositions include U.S. Pat. Nos. 3,729,313, 3,741,769, 3,808,006, 4,228,232, 4,250,053 and 4,428,807; H. J. Timpe and H. Baumann, Wiss. Z. Tech. Hochsch. Leuna-Merseburg, 26, 439 (1984); H. Baumann, B. Strehmel, H. J. Timpe and U. Lammel, J. Prakt. Chem, 326 (3), 415 (1984); and H. Baumann, U. Oertel and H. J. Timpe, Euro. Polym. J., 22, 313 (1986).
Photopolymerizable compositions usually contain a photoinitiator system, a polymerizable monomer, and optionally adjuvants. Often pigments are added to color or opacify the compositions. The addition of pigments usually inhibits the initiation of polymerization because of competitive light absorption or light scattering, which decreases the amount of light available for absorption by the photoinitiator system. It would be desirable to be able to use pigments as part of a photoinitiator system in a photopolymerizable composition. In the present invention, insoluble sensitizers can serve both as sensitizers and colorants.
A three component system describing the use of iodonium salts, electron donor compound, and soluble sensitizer is described in U.S. Ser. Nos. 34,065 and 34,066, both filed Apr. 2, 1987.
However, these applications do not suggest the use of an insoluble pigment as a sensitizer.
U.S. Pat. No. 4,257,915 discloses semiconductors in combination with an oxidation-reduction system for producing images. This patent does not teach the use of iodonium salts or organic pigments.